1. Field of the Invention
The present invention relates to a pump casing manufactured by such as press forming of steel plate, and in particular, to a pump casing advantageously used for a centrifugal pump.
2. Prior Arts
A conventional pump casing manufactured by press forming of steel plate usually includes an outer casing of a cylindrical shape and a guide vane or a volute member which is separately fabricated and disposed inside of the outer casing to provide a fluid path.
FIG. 8(a) is a longitudinal sectional view of a pump of prior art, and FIG. 8(b) is a sectional view taken on line A--A of FIG. 8(a). In this conventional pump, the volute casing defines a volute room 4 which guides fluid delivered from an impeller 1 to an discharge port 11. The volute room 4 is composed of side plates 5 and 6 disposed at each side, a peripheral plate 7 provided along the periphery of the side plates 5 and 6, and a volute vane or volute member 8 disposed in a space defined by the side plates and the peripheral plate and extended in a circumferential direction along a predetermined distance. These side plates 5 and 6, the peripheral plate 7 and the volute vane 8 are all manufactured by press forming of metal plates. In going from a starting point 8a of the volute to a predetermined point 8b, the volute casing is radially deviated outwardly, thereby increasing the cross-sectional area of the fluid path within the volute casing in going towards the discharge port. Beyond the predetermined point 8b, the volute casing is deviated in an axial direction as denoted by 5a for increasing the cross-sectional area of the fluid path in going towards the discharge port 11. Thus, the cross-sectional area of the fluid path is gradually increased from the line I--I to line IV--IV [FIG. 8(b)].
In the above-mentioned prior art, since the side plates 5 and 6, the peripheral plate 7, the volute vane 8 and an outer plate 9 are all manufactured by press forming of metal plates, the casing can be manufactured with easier fabrication and lower cost in comparison with the manufacturing method of casting. Further, by deviating the volute fluid path in both radial and axial directions, it becomes possible to decrease the radial dimension of the pump to some extent, and to thereby decrease the cost.
In the above-mentioned prior arts, however, since the volute vane 8 is disposed inside of the pump casing, the outside diameter of the outer casing inevitably become greater. In consequence, there is a problem that it is required to increase the thickness of the casing to increase the strength against the internal fluid pressure, and to reinforce the casing to prevent deformation of the side walls of the casing. When the volute portion is expanded in an axial direction to decrease the radial excessive space required for arranging the volute vane 8 outside of the impeller as mentioned above, the pump performance deteriorates.
Further, in the above-mentioned cases, since the shape of the fluid path is not smooth at the region where the fluid flows out from inside of the fluid path to outside of the same, two problems are caused, generation of noise and deterioration of efficiency.